Resistive random access memory (RRAM) is a new non-volatile type memory based on dielectric materials. The memory element in an RRAM cell is a resistive switching device. For a resistive switching device, for example, if a sufficiently high voltage of one polarity is applied, the resistive switching device can switch (SET) to a low-resistance state. On the other hand, if a sufficiently high voltage of the opposite polarity is applied, the resistive switching device can switch (RESET) to a high-resistance state. The different resistance states of the resistive switching device can be used to represent “0” and “1,” respectively.
Due to its high-speed random access, low power consumption, high cyclability, high retention at high temperature, and scalability to 10 nm and below, RRAM has become a promising non-volatile memory technology to potentially replace NOR and NAND flash memories and dynamic random access memory (DRAM).
In order for an RRAM to operate properly, a controlling element having variable resistance needs to be serially connected to the resistive switching device, so that a current passing through the RRAM cell can be regulated separately for different operations, such as READ, SET, or RESET. Due to the need for operation voltages in opposite polarities, a single p-n junction or a single Schottky diode is not suitable for use as the controlling element. A conventional RRAM cell typically includes a resistive switching device and a transistor, such as a field effect transistor (FET), as the variable resistance controlling element. The resistance of an FET between its source and drain can be controlled by changing the voltage applied to the gate of the FET. Thus, if a resistive switching device is electrically connected to the source or drain of the FET, the resistance serially connected to the resistive switching device can be regulated by controlling the voltage applied to the gate of the FET. However, since the FET is a planar device, it cannot be vertically integrated with the resistive switching device. Therefore, a conventional RRAM employing the FET as the variable resistance controlling element may have a large cell size that makes high storage density and high capacity difficult to achieve.
Accordingly, there is a need for an improved RRAM.